A Steady-State and Picosecond Pump-Probe Investigation of the Photophysics of an Acyl and a Bis(acyl)phosphine Oxide

The photophysics (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (1) and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (4) have been investigated by fluorescence, phosphorescence, and low temperature time resolved electron spin resonance. Both 1 and 4 undergo α-cleavage to produce benzoyl and phosphorous centered radicals. The photochemistry of 1 and 4 has been investigated by nanosecond laser flash photolysis, picosecond pump-probe spectroscopy, and steady-state photolysis. The singlet states of 1 and 4 and the phosphorous centered radicals produced by α-cleavage were characterized directly by time resolved absorption spectroscopy. The triplet states of 1 and 4 were characterized indirectly by quenching with 1-phenylnaphthalene as a selective triplet quencher. The use of 1-phenylnaphthalene indicates that α-cleavage occurs mainly from the triplet states of 1 and 4. However, the observed rate of formation of phosphorous centered radicals derived from picosecond investigations is experimentally indistinguishable from the rate of disappearance of the singlet states of 1 and 4. The results are compatible with mechanisms for which the rate of intersystem crossing of the S1 states of 1 and 4 limits the observed rate of α-cleavage, because the rate of α-cleavage is of the same order or faster than the rate of intersystem crossing. This relatively uncommon situation appears to have an analogy in the well investigated photochemistry of dibenzyl ketone.